In recent years, tendency of a shorter wavelength of laser beam as a light source which has been used to record and/or reproduce information onto/from optical discs, has become a main stream. For example, a blue-violet semiconductor laser, a blue-SHG laser performing wavelength conversion of an infrared semiconductor laser utilizing a second harmonic wave, etc., having 400-420 nm wavelength have been made practical.
It becomes possible to record information of 15-20 GB onto an optical disc having a diameter of 12 cm by using a blue-violet laser source and a objective lens having NA (Numerical aperture) which is the same as a DVD (Digital Versatile Disc). When NA is 0.85, 23-25 GB information can be recorded onto the optical disc having a diameter of 12 cm. In this specification, the optical disc and an optical-magnetic disc using a blue-violet laser light source are called “a high density optical disc”.
At this moment, two industrial standards for the high density optical disc have been proposed. One is BLU-RAY DISC (it will be called BD hereinafter) having a thickness of 0.1 mm where an objective lens having a NA 0.85, is used, and the other is HD DVD (it will be called HD hereinafter) having a thickness of 0.6 mm where the objective lens having a NA 0.65-0.67 is used. A high density optical disc player/recorder capable of recording and/or reproducing both high density discs will be necessary based on an assumption that these two high density discs based on these two industrial standards will become popular in a market in future.
It is not enough for value of an optical disc player and/or recorder to be able to record/reproduce information onto/from only a high density disc. Taking account that DVDs and CDs (compact discs) on which various kinds of information is recorded are on a marketplace, it is not enough for value of the optical disc player/recorder to be able to record/reproduce information onto/from only a high density optical disc. In order to raise a product value, it is necessary for an optical disc player/recorder to record/reproduce information not only onto/from a high density disc but also a DVD and a CD which users posses. From these backgrounds, an optical pickup for an optical disc player/recorder for high density optical discs is required to be able to appropriately record/reproduce information onto/from a high density optical disc, DVD and CD.
However, since BD is different from HD in terms of specifications such as numerical aperture and protective substrate thickness, if a common objective lens is used for conducting recording and/or reproducing of information compatibly for BD and HD, a degree of freedom for design of the objective lens is restricted, resulting in a problem of deteriorations in temperature characteristics. In contrast to this, TOKUKAI No. 2004-295983 discloses an optical pickup apparatus that can conduct recording and/or reproducing of information compatibly while securing a degree of freedom for design, by providing two objective lenses respectively for BD and HD.
However, in the optical pickup apparatus shown in TOKUKAI No. 2004-295983, there is a problem that the optical pickup apparatus is complicated in terms of a structure and is large in a size, because separate light sources are provided for respective two objective lenses.